The enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture. Delayed stall functions during the translational portions of the stroke, when the wings sweep through the air with a large angle of attack. In contrast, rotational circulation and wake capture generate aerodynamic forces during stroke reversals, when the wings rapidly rotate and change direction. In addition to contributing to the lift required to keep an insect aloft, these two rotational mechanisms provide a potent means by which the animal can modulate the direction and magnitude of flight forces during steering maneuvers. A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.

https://science.sciencemag.org/content/sci/284/5422/1954.full.pdf

Wing rotation and the aerodynamic basis of insect flight.

Wing rotation and the aerodynamic basis of insect flight

The aim of this review is to consider the potential benefits that females may gain from mating more than once in a single reproductive cycle. The relationship between non-genetic and genetic benefits is briefly explored. We suggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begin by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple mating can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifying cases where females use pre-copulatory cues to identify mates prior to remating. In the simplest case, females remate because they identify a superior mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre-copulatory cues may be less reliable and females mate with several males to promote post-copulatory mechanisms that bias paternity. Although a distinction is drawn between sperm competition and cryptic female choice, we point out that the genetic benefits to polyandry in terms of producing more viable or sexually attractive offspring do not depend on the exact mechanism that leads to biased paternity. Post-copulatory mechanisms of paternity biasing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those that improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third possibility is that genetic diversity among offspring is directly favoured. This can be due to bet-hedging (due to mate assessment errors or temporal fluctuations in the environment), beneficial interactions between less related siblings or the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevailing environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among progeny in elevating fitness. We conclude that post-copulatory mechanisms provide a more reliable way of selecting a genetically compatible mate than pre-copulatory mate choice. Some of the best evidence for cryptic female choice by sperm selection is due to selection of more compatible sperm. Two future areas of research seem likely to be profitable. First, more experimental evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promoting assortative fertilization and increasing reproductive isolation between populations may help us to understand sympatric speciation.

Why do females mate multiply? A review of the genetic benefits.

Recent advances in integrative studies of locomotion have revealed several general principles. Energy storage and exchange mechanisms discovered in walking and running bipeds apply to multilegged locomotion and even to flying and swimming. Nonpropulsive lateral forces can be sizable, but they may benefit stability, maneuverability, or other criteria that become apparent in natural environments. Locomotor control systems combine rapid mechanical preflexes with multimodal sensory feedback and feedforward commands. Muscles have a surprising variety of functions in locomotion, serving as motors, brakes, springs, and struts. Integrative approaches reveal not only how each component within a locomotor system operates but how they function as a collective whole.

https://science.sciencemag.org/content/sci/288/5463/100.full.pdf

How Animals Move: An Integrative View

The flight of insects has fascinated physicists and biologists for more than a century. Yet, until recently, researchers were unable to rigorously quantify the complex wing motions of flapping insects or measure the forces and flows around their wings. However, recent developments in high-speed videography and tools for computational and mechanical modeling have allowed researchers to make rapid progress in advancing our understanding of insect flight. These mechanical and computational fluid dynamic models, combined with modern flow visualization techniques, have revealed that the fluid dynamic phenomena underlying flapping flight are different from those of non-flapping, 2-D wings on which most previous models were based. In particular, even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing-wake interactions following stroke reversal. This progress has enabled the development of simple analytical and empirical models that allow us to calculate the instantaneous forces on flapping insect wings more accurately than was previously possible. It also promises to foster new and exciting multi-disciplinary collaborations between physicists who seek to explain the phenomenology, biologists who seek to understand its relevance to insect physiology and evolution, and engineers who are inspired to build micro-robotic insects using these principles. This review covers the basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena.

/pdf/the-aerodynamics-of-insect-flight-4ladwfpmp7.pdf

The aerodynamics of insect flight

INSECTS cannot fly, according to the conventional laws of aerodynamics: during flapping flight, their wings produce more lift than during steady motion at the same velocities and angles of attack1–5. Measured instantaneous lift forces also show qualitative and quantitative disagreement with the forces predicted by conventional aerodynamic theories6–9. The importance of high-life aerodynamic mechanisms is now widely recognized but, except for the specialized fling mechanism used by some insect species1,10–13, the source of extra lift remains unknown. We have now visualized the airflow around the wings of the hawkmoth Manduca sexta and a 'hovering' large mechanical model—the flapper. An intense leading-edge vortex was found on the down-stroke, of sufficient strength to explain the high-lift forces. The vortex is created by dynamic stall, and not by the rotational lift mechanisms that have been postulated for insect flight14–16. The vortex spirals out towards the wingtip with a spanwise velocity comparable to the flapping velocity. The three-dimensional flow is similar to the conical leading-edge vortex found on delta wings, with the spanwise flow stabilizing the vortex.

Leading-edge vortices in insect flight

Dimensionless numbers are important in biomechanics because their constancy can imply dynamic similarity between systems, despite possible differences in medium or scale. A dimensionless parameter that describes the tail or wing kinematics of swimming and flying animals is the Strouhal number, St = fA/U, which divides stroke frequency (f) and amplitude (A) by forward speed (U). St is known to govern a well-defined series of vortex growth and shedding regimes for airfoils undergoing pitching and heaving motions. Propulsive efficiency is high over a narrow range of St and usually peaks within the interval 0.2 < St < 0.4 (refs 3-8). Because natural selection is likely to tune animals for high propulsive efficiency, we expect it to constrain the range of St that animals use. This seems to be true for dolphins, sharks and bony fish, which swim at 0.2 < St < 0.4. Here we show that birds, bats and insects also converge on the same narrow range of St, but only when cruising. Tuning cruise kinematics to optimize St therefore seems to be a general principle of oscillatory lift-based propulsion.

Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency

Insect wings are complex structures that deform dramatically in flight. We analyzed the aerodynamic consequences of wing deformation in locusts using a three-dimensional computational fluid dynamics simulation based on detailed wing kinematics. We validated the simulation against smoke visualizations and digital particle image velocimetry on real locusts. We then used the validated model to explore the effects of wing topography and deformation, first by removing camber while keeping the same time-varying twist distribution, and second by removing camber and spanwise twist. The full-fidelity model achieved greater power economy than the uncambered model, which performed better than the untwisted model, showing that the details of insect wing topography and deformation are important aerodynamically. Such details are likely to be important in engineering applications of flapping flight.

https://science.sciencemag.org/content/sci/325/5947/1549.full.pdf

Details of Insect Wing Design and Deformation Enhance Aerodynamic Function and Flight Efficiency

Flying insects generate forces that are too large to be accounted for by conventional steady-state aerodynamics. To investigate these mechanisms of force generation, we trained red admiral butterflies, Vanessa atalanta, to fly freely to and from artificial flowers in a wind tunnel, and used high-resolution, smoke-wire flow visualizations to obtain qualitative, high-speed digital images of the air flow around their wings. The images show that free-flying butterflies use a variety of unconventional aerodynamic mechanisms to generate force: wake capture, two different types of leading-edge vortex, active and inactive upstrokes, in addition to the use of rotational mechanisms and the Weis-Fogh 'clap-and-fling' mechanism. Free-flying butterflies often used different aerodynamic mechanisms in successive strokes. There seems to be no one 'key' to insect flight, instead insects rely on a wide array of aerodynamic mechanisms to take off, manoeuvre, maintain steady flight, and for landing.

Unconventional lift-generating mechanisms in free-flying butterflies.

SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization,
that dragonflies fly by using unsteady aerodynamic mechanisms to generate
high-lift, leading-edge vortices. In normal free flight, dragonflies use
counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing
downstroke, attached flow on the forewing upstroke, and attached flow on the
hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats
with highly separated downstroke flows, with a single LEV attached across both
the fore- and hindwings. We use smoke visualizations to distinguish between
the three simplest local analytical solutions of the Navier–Stokes
equations yielding flow separation resulting in a LEV. The LEV is an open
U-shaped separation, continuous across the thorax, running parallel to the
wing leading edge and inflecting at the tips to form wingtip vortices. Air
spirals in to a free-slip critical point over the centreline as the LEV grows.
Spanwise flow is not a dominant feature of the flow field – spanwise
flows sometimes run from wingtip to centreline, or vice versa –
depending on the degree of sideslip. LEV formation always coincides with rapid
increases in angle of attack, and the smoke visualizations clearly show the
formation of LEVs whenever a rapid increase in angle of attack occurs. There
is no discrete starting vortex. Instead, a shear layer forms behind the
trailing edge whenever the wing is at a non-zero angle of attack, and rolls
up, under Kelvin–Helmholtz instability, into a series of transverse
vortices with circulation of opposite sign to the circulation around the wing
and LEV. The flow fields produced by dragonflies differ qualitatively from
those published for mechanical models of dragonflies, fruitflies and
hawkmoths, which preclude natural wing interactions. However, controlled
parametric experiments show that, provided the Strouhal number is appropriate
and the natural interaction between left and right wings can occur, even a
simple plunging plate can reproduce the detailed features of the flow seen in
dragonflies. In our models, and in dragonflies, it appears that stability of
the LEV is achieved by a general mechanism whereby flapping kinematics are
configured so that a LEV would be expected to form naturally over the wing and
remain attached for the duration of the stroke. However, the actual formation
and shedding of the LEV is controlled by wing angle of attack, which
dragonflies can vary through both extremes, from zero up to a range that leads
to immediate flow separation at any time during a wing stroke.

/pdf/dragonfly-flight-free-flight-and-tethered-flow-35vxeiljgv.pdf

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

Adrian L. R. Thomas

Papers

Leading-edge vortices in insect flight

Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency

Details of Insect Wing Design and Deformation Enhance Aerodynamic Function and Flight Efficiency

Unconventional lift-generating mechanisms in free-flying butterflies.

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.